TUESDAY, June 16 (HealthDay News) -- Black women are twice as likely to give birth prematurely as white women, and a greater likelihood of depression may play a role in that, a new study suggests.

Researchers looked at birth-rate data collected over six years from the Coronary Artery Risk Development in Young Adults Study, which included health information from about 5,000 young adults living in four metropolitan areas.

Of the 555 women who gave birth between 1990 and 1996 in the larger study, 18.1 percent of black women gave birth prematurely compared with 8.5 percent of white women, according to the study, which appears in the June online issue of the Journal of Women's Health.

Premature birth was defined as any birth before 37 weeks of gestation. Normal gestation ranges from 38 to 42 weeks.

"Preterm births are one of the most significant health disparities in the United States, and the overall number of these births increased from 10.6 percent in 2000 to 12.8 percent in 2005," said the study's lead author, Amelia Gavin, a University of Washington assistant professor of social work.

Researchers aren't sure of the reasons for the disparity, but pre-pregnancy depression could a role, Gavin said. Women who were depressed before becoming pregnant had a greater likelihood of giving birth prematurely.

Yet researchers can't say with certainty that depression is causing the preterm deliveries or what impact depression during pregnancy might have on gestation because the data did not include that information.

"At this point we can't say that pre-pregnancy depressive mood is a cause of preterm birth or how race affects this association," Gavin said. "But it seems to be a risk factor in giving birth prematurely, and higher pre-pregnancy depressive mood among black women compared to white women may indirectly contribute to the greater odds of preterm birth found among black women."

Another possibility is that the higher preterm birth rate among black women might be the result of "weathering," or accelerated declines in health because of socioeconomic or other factors, she said.

"What some people experience by being black takes a toll on the physiological system, and over time, wear and tear that occurs across neural, neuroendocrine and immune systems as a result of chronic exposure to stressors lead to health disparities for blacks," Gavin said. "Some of this may manifest itself in premature birth and low-birth weight."